The White House has criticized Russia for its "red carpet" treatment of Syrian President Bashar al-Assad, who made a rare visit to Moscow to see President Vladimir Putin.

"We view the red carpet welcome for Assad, who has used chemical weapons against his own people, at odds with the stated goal by the Russians for a political transition in Syria," White House spokesman Eric Schultz told reporters on October 21.

Assad flew to Moscow in the evening on October 20 to thank Putin personally for his military support, in a surprise visit that Russian state media cast as a diplomatic coup.

It was Assad's first foreign visit since the start of the Syrian crisis in 2011, and came three weeks after Russia launched a campaign of air strikes against Islamist militants and rebels in Syria that has bolstered Assad's forces.

"We thank you for standing by Syria's territorial integrity and its independence," Assad, who was shown smiling as he shook hands with Putin and other officials, told the Russian leader.

Putin, who said he had invited Assad, thanked the Syrian president for "coming to Moscow despite a tragic situation in your country."

Citing the presidency, Syrian state radio on October 21 reported that Assad had already returned to Damascus.

Russia, a long-standing military ally of Syria, launched a bombardment campaign in support of Assad late last month and insists he must stay in power -- rejecting the view of U.S. and Western officials who say the only diplomatic solution to Syria's civil war would require Assad to leave office.

The United States and its allies have accused Moscow of targeting armed Syrian groups opposed to Assad, some of whom are U.S.-backed.

After the visit, Russia said Foreign Minister Sergei Lavrov and U.S. Secretary of State John Kerry had agreed to meet with their Turkish and Saudi counterparts in Vienna on October 23 to discuss the crisis in Syria.

The ministry said Lavrov and Kerry spoke by telephone on October 21 and that the two focused on "the situation in Syria in the context of preparations for the meeting" in Vienna.

Meanwhile, the Britain-based Syrian Observatory for Human Rights, which taps network of activists inside Syria for information, said the Russian bombardments had killed more than 120 civilians.

Kremlin spokesman Dmitry Peskov said the talks between Putin and Assad on October 20 focused on the joint military campaign by Russia and Syria against "terrorist groups" and the continuation of Russian air strikes in Syria.

He said Assad informed Putin about the plans of Syrian government forces.

Russian television showed footage of Putin, together with Foreign Minister Sergei Lavrov and Defense Minister Sergei Shoigu, meeting with Assad and one of his top advisers.

The Kremlin said on October 21 that Putin told Assad that about 4,000 people with origins in former Soviet republics are now fighting against Syrian government troops.

The Kremlin also said Putin told Assad that Moscow cannot allow those people to return to Russia after they acquire combat experience in Syria and undergo "indoctrination" by Islamic militants.

In video footage broadcast by Russian media on October 21, Assad told Putin that "terrorism" had spread through the region but that it "would have taken much larger areas and would have spread throughout much larger territory if it were not for your actions and your decisions."

A U.S.-led coalition that includes Bahrain, Jordan, Qatar, Saudi Arabia, and the United Arab Emirates began air strikes in September 2014 that have been targeting Islamic State (IS) militants and Al-Qaeda-linked fighters from the Khorasan group and the Al-Nusra Front.

Moscow has claimed that three weeks of Russian air strikes have been focusing on IS militants and other international terrorists operating in Syria.

But the United States says Russia's attacks have mostly targeted moderate Syrian opposition fighters who have been trained and armed by the West and are fighting against IS and Al-Qaeda, as well as against Assad's regime.

Washington says that means Russia's air strikes have bolstered Assad's regime and made it more difficult for a political solution to Syria's four-year-old civil war to be negotiated diplomatically.

But a statement on the Syrian presidency's official Facebook page on October 21 quoted Assad as saying that "terrorism is an obstacle to a political solution."

Assad's regime and the Kremlin consider all forces that are fighting against Syrian government troops to be terrorists.

Assad's statement on Facebook said the aim of military cooperation between Damascus and Russia is to "eradicate terrorism."

The Syrian Observatory for Human Rights says that the hundreds of Russian air strikes in Syria over the past three weeks have killed at least 370 people, including 127 civilians. The dead civilians include 36 children and 34 women, it says.

On October 20, Reuters quoted a senior pro-government military source in Syria as saying that three Russians had been killed while alongside Assad's forces when a shell hit their position in the coastal province of Latakia.

The Russian Defense Ministry swiftly denied the claim, saying that "there have been no casualties among soldiers from Russia's armed forces in Syria."

Russian Foreign Ministry spokeswoman Maria Zakharova on October 21 accused Western journalists of failing to contact the ministry about the purported deaths and of carrying out a campaign of "information aggression" against Russia with its coverage of Moscow's military action in Syria.

Putin's talks with Assad came on the same day the Pentagon announced that Moscow and Washington had signed an agreement to minimize risks of incidents between Russian and U.S.-led coalition aircraft in Syrian airspace.

That agreement sets up radio frequencies and a telephone hotline for direct communications between Russian and U.S. military forces.

Pentagon spokesman Peter Cook said the agreement does not include zones of cooperation or the sharing of target information.